In communication systems which accept dual input signals, cross-interference between such signals is often high and suitable reception thereof tends to deteriorate because of the interference problem. A typical system may be, for example, a microwave line-of-sight communications link, the capacity of which is doubled by transmitting independent data streams which are orthogonally polarized. Under normal circumstances cross-polarization effects, i.e. the effects of interference between such orthogonally polarized signals, are sufficiently low to result in overall good performance. However, when poor propagation conditions exist, such as during heavy rain, or when air turbulence is present, or when the signals travel through a multipath communication medium, the cross-polarization effects tend to increase and the two signals are not easily isolated merely on the basis of polarization.
Accordingly, appropriate signal processing techniques must be utilized to cancel the cross-polarization effects that arise in the dual signal channel and, thereby, to reduce the interference between them to an acceptable level. For example, under typical normal conditions, cross-polarization levels may be as low as 40 db below the principal signal components. However, in the presence of a relatively heavy rainstorm the principal signal components may fade considerably while the cross-polarized signal components may be increased in strength by a relatively large amount so that the latter components are less than 10 db below the principal signal components and the interference becomes intolerable.
While it is possible under such conditions to turn off one of the transmitter channels, thereby removing the source of the cross-polarization interference, such a solution is not desirable since it reduces the signal carrying capacity of the system in half.
Another proposed system for dealing with cross-polarization effects is described in U.S. Pat. No. 3,500,207 issued on Mar. 10, 1970, to Clyde I. Ruthroff. The Ruthroff system merely corrects for a polarization alignment by transmitting a pilot signal in one of the cross-polarizations which is then detected as an error signal in the other polarization at the receiver and fed back to a polarization rotator system which rotates the entire received signal in order to minimize the error. Such a system merely rotates both of the received signal components simultaneously to an appropriate orientation which tends to produce the least cross-polarization effect, both components, however, remaining substantially orthogonal with respect to each other. However, should the orthogonality change as a result of the transmission, as is likely to happen under poor propagation conditions, the mere rotation of such components will not provide the optimum elimination of cross-polarization effects which may have occurred in the transmission medium.
Another approach is discussed in U.S. Pat. No. 3,735,266 issued on May 22, 1973, to N. Amitay. In Amitay's system, a pilot signal is transmitted with each of the cross-polarized channels and means are included in the receiver for detecting components of the pilot signal from each channel to indicate the level of cross-polarization which occurs. The components of such pilot signals are then appropriately processed to generate control signals, proportional to the degree of correction required, to cancel the cross-polarization component in each of the channels. Cancellation is achieved by control circuits, either in the RF or the IF sections of the receiver, in which the received signals are directly operated upon by the control signals to cancel the cross-polarization components therein.
The Amitay system does not consider the cross-polarization effects over the entire bandwidth of the signal but rather only considers the effect which occurs substantially at or near the center frequency of the frequency band which is being transmitted, i.e. at the region where the pilot signal is present. Moreover, such a system requires the pilot signal to be carried along with the information signal and not only necessitates the use of extra equipment both at the transmitter and the receiver, but also reduces the amount of information that can be carried by the system. Further, the receiver system of Amitay is not decision-directed, i.e. the cross-polarization correction is not self-contained in the receiver system but requires not only the generation of a pilot signal at the transmitter system to be carried along with the information signal but also further requires correction calculations which are based on a pre-conceived, or predicted, transmission channel model and on the assumption that all components of the system are ideal components having no inherent error generating characteristics. Such a system can be contrasted with decision-directed systems which not only avoid the need for a pilot signal but also minimize system errors no matter where they occur. Thus, the decision-directed system considers transmission effects, no matter what the transmission characteristics are (they need not be known or predicted ahead of time) and also considers the effects of using non-ideal, or imperfect, components in the system.
Decision-directed systems have been suggested for use, for example, in diversity communication systems, such as described in U.S. Pat. No. 3,879,664, issued on Apr. 22, 1975, to Peter Monsen, and U.S. Pat. No. 3,633,107, issued on Jan. 4, 1972, to D. M. Brady. In such systems the same signal is received at different diversity channel input receivers and a decision-directed generation of an error signal is appropriately determined and applied to filter equalizer circuitry within each channel to minimize the errors which arise during transmission through a dispersive transmission medium. Such systems do not, however, involve the problem of cross-polarization as would occur in a dual channel system wherein two transmitters transmit signals which are polarized with respect to each other (usually orthogonally polarized) and two receivers are used to receive the two polarized signals. The latter case, in which the problem of interference between the cross-polarized signals arises, requires solutions not previously suggested in any of the above-discussed patents. It is desirable that a suitable decision-directed control system for minimizing errors be devised so that a maximum amount of information can be transmitted by a polarized transmission system without the need for carrying reference signals along with the information signals. The present invention adapts principles related to the use of filter equalization techniques to the cross-polarization problem to provide an effecive solution thereto.